listen-notify-backend

backend.py

from eventlet.hubs import trampoline

def start_listening(queue):
    """Connects to PgSQL and listens to notifications, inspired by http://initd.org/psycopg/articles/2010/12/01/postgresql-notifications-psycopg2-eventlet/"""
    if not queue:
        return
    connection = psycopg2.connect(DSN)
    cursor = connection.cursor()
    cursor.execute("LISTEN field_value_change")

    while True:
        trampoline(connection, read=True)
        connection.poll()
        while connection.notifies:
            notify = connection.notifies.pop()
            queue.put(json.loads(notify.payload))
            
def emitter(queue):
    """Listens to queued events and emits data_changed broadcast event to a socket"""
    while True:
        element = queue.get()
        socketio.emit('data_changed', {'data': {'name': element['field'], 'value': element['value']}}, broadcast=True)

# ...

dbThread = None
qThread = None
        
@socketio.on('connect')
def connect():
    """On first socket.io connection registers the database listener and value emitter"""
    global dbThread
    global qThread
    q = eventlet.Queue()
    if dbThread is None:
        dbThread = socketio.start_background_task(target=start_listening, queue=q)
        qThread = socketio.start_background_task(target=emitter, queue=q)